Australia's Deputy Opposition leader Julie Bishop says Prime Minister Julia Gillard still needs to "prove her innocence" over her alleged involvement in the establishment of a legal entity set up by two union officials 20 years ago.

Bishop spent the final week of the parliamentary year quizzing Gillard over the advice she gave former Australian Workers' Union figures Ralph Blewitt and Bruce Wilson, who set up the AWU Workplace Reform Association in 1992.

Gillard has denied any wrongdoing and has challenged the opposition to come up with evidence she acted illegally or inappropriately.

On Macquarie Radio on Friday, Bishop repeated opposition calls for a judicial inquiry, saying Gillard needed to "prove her innocence".

"Presumably part of the deal to leave Slater & Gordon was to hand over her practising certificate and never practise law again," Bishop said.

Gillard voluntarily left Slater & Gordon after questions arose over setting up the association.